Method of baking ceramic honeycomb structure

ABSTRACT

In the method for firing of ceramic honeycomb structure according to the present invention, when a formed honeycomb material having an apparent volume of 5 liters or more is fired in a firing atmosphere, the heating of the firing atmosphere from a temperature (180 to 300° C.) at which the organic binder contained in the formed honeycomb material begins to burn, to 300° C. is conducted at a temperature elevation rate of +25 (° C./hr) or more and also at such a temperature elevation rate that the difference of the temperature of the central portion of the formed honeycomb material from the temperature of the firing atmosphere is kept in a given temperature range.

TECHNICAL FIELD

The present invention relates to a method for firing of ceramichoneycomb structure, which comprises firing a formed honeycomb materialto obtain a ceramic honeycomb structure, particularly to a method forfiring of ceramic honeycomb structure, which, when a large-sized formedhoneycomb material having an apparent volume of 5 liters or more isfired, can effectively prevent firing defects such as cracks and thelike and further can shorten the conventional firing time strikingly.

BACKGROUND ART

In various fields of chemistry, electric power, steel, industrial wastedisposal, etc., ceramic honeycomb structures superior in heat resistanceand corrosion resistance are in use as a dust-collecting filter forpurposes such as environmental protection (e.g. pollution control),product recovery from high-temperature gas, and the like. For example, adiesel particulate filter (DPF) for capture of particulates emitted froma diesel engine is used under severe conditions of high-temperature,corrosive gas atmosphere; therefore, as the DPF, there is being used aceramic honeycomb structure preferably.

In particular, a ceramic honeycomb structure of low pressure loss andhigh porosity has been required in recent years in order to allow thedust-collecting filter to have a higher treatment capability. Forproduction of such a ceramic honeycomb structure of high porosity, thereis disclosed, in, for example, JP-A-2002-219319, a process forproduction of porous honeycomb filter, which comprises kneading a rawmaterial for aggregate particles (e.g. a raw material to becomecordierite), water, a binder (an organic binder such as methyl celluloseor the like), a hole-making agent (an organic substance such as graphiteor the like), etc. to form a plastic puddle, forming the plastic puddleto obtain a formed material, and drying and firing the formed material.

In such a process for producing a honeycomb structure using a binder,however, there has been a problem that, during the firing of the formedmaterial, there appear firing defects such as crack and the like. Thereason for the problem is considered to be that, during the firing ofthe formed material, a large temperature difference arises between thecentral portion of the formed material where a sharp temperature riseappears owing to the burning of the binder contained in the formedmaterial and the peripheral portion of the formed material where atemperature change takes place so as to approximately follow thetemperature of the firing atmosphere and, as a result, a thermal stressgenerates inside the formed material. Hence, there was taken, as seenin, for example, JP-A-1-249665, a countermeasure of employing a minimumtemperature elevation rate (i.e. slow heating) in a temperature range(ordinarily, 180 to 300° C.) in which the binder contained in the formedmaterial burns and, thereby, enabling slow burning of the binder andconducting firing so that there arises no temperature difference betweenthe central portion of the formed material and the peripheral portion ofthe formed material.

Firing defects such as crack and the like can be prevented when acountermeasure such as mentioned above is taken. However, there has beena problem that the firing time is longer. A longer firing time is notpreferred from the standpoints of energy consumed and productivity.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the above problems andaims at providing a method for firing of ceramic honeycomb structure,which has advantages of being able to prevent firing defects such ascrack and the like effectively and further to shorten the conventionalfiring time strikingly.

The present inventor made a study in order to achieve the above aim and,as a result, found that the aim can be achieved by, in firing a formedhoneycomb material in a firing atmosphere, conducting the heating of thefiring atmosphere from a temperature (180 to 300° C.) at which theorganic binder contained in the formed honeycomb material begins toburn, to 300° C., at a temperature elevation rate of +25 (° C./hr) ormore and also at such a temperature elevation rate that the differenceof the temperature of the central portion of the formed honeycombmaterial from the temperature of the firing atmosphere is kept in agiven range. The present invention has been completed based on the abovefinding. Hence, the present invention provides the following methods forfiring of ceramic honeycomb structure.

[1] A method for firing of ceramic honeycomb structure, which comprisesmixing and kneading at least a raw material for aggregate particles,water and an organic binder to form a puddle, forming the puddle into ahoneycomb shape having a plurality of cells (to become passages forfluid) divided by partition walls, drying the honeycomb shape to obtaina formed honeycomb material having an apparent volume of 5 liters ormore and a partition wall thickness of 90 μm or more, and firing theformed honeycomb material in a firing atmosphere which is heated at agiven temperature elevation rate to obtain a ceramic honeycombstructure, wherein, in the firing of the formed honeycomb material inthe firing atmosphere, the heating of the firing atmosphere from atemperature (180 to 300° C.) at which the organic binder contained inthe formed honeycomb material begins to burn, to 300° C. is conducted ata temperature elevation rate of +25 (° C./hr) or more and also at such atemperature elevation rate that the difference of the temperature of thecentral portion of the formed honeycomb material from the temperature ofthe firing atmosphere is kept in a range of −80° C. to +80° C.

[2] A method for firing of ceramic honeycomb structure, which comprisesmixing and kneading at least a raw material for aggregate particles,water and an organic binder to form a puddle, forming the puddle into ahoneycomb shape having a plurality of cells (to become passages forfluid) divided by partition walls, drying the honeycomb shape to obtaina formed honeycomb material having an apparent volume of 5 liters ormore and a partition wall thickness of less than 90 μm, and firing theformed honeycomb material in a firing atmosphere which is heated at agiven temperature elevation rate to obtain a ceramic honeycombstructure, wherein, in the firing of the formed honeycomb material inthe firing atmosphere, the heating of the firing atmosphere from atemperature (180 to 300° C.) at which the organic binder contained inthe formed honeycomb material begins to burn, to 300° C. is conducted ata temperature elevation rate of +25 (° C./hr) or more and also at such atemperature elevation rate that the difference of the temperature of thecentral portion of the formed honeycomb material from the temperature ofthe firing atmosphere is kept in a range of −40° C. to +50° C.

[3] A method for firing of ceramic honeycomb structure, which comprisesmixing and kneading at least a raw material to become cordierite andwater to form a puddle, forming the puddle into a honeycomb shape havinga plurality of cells (to become passages for fluid) divided by partitionwalls, drying the honeycomb shape to obtain a formed honeycomb materialhaving an apparent volume of 5 liters or more, and firing the formedhoneycomb material in a firing atmosphere which is heated at a giventemperature elevation rate to obtain a ceramic honeycomb structure,

wherein, in the firing of the formed honeycomb material in the firingatmosphere, the heating of the firing atmosphere from 800° C. to 1,000°C. is conducted at a temperature elevation rate of +40 (° C./hr) orless.

[4] A method for firing of ceramic honeycomb structure according to theabove [3], wherein, when the formed honeycomb material obtained has apartition wall thickness of 90 μm or more, the heating of the firingatmosphere from 800° C. to 1,000° C. in the firing of the formedhoneycomb material is conducted at such a temperature elevation ratethat the difference of the temperature of the central portion of theformed honeycomb material from the temperature of the firing atmosphereis kept in a range of −60° C. to +60° C.

[5] A method for firing of ceramic honeycomb structure according to theabove [3], wherein, when the formed honeycomb material obtained has apartition wall thickness of less than 90 μm, the heating of the firingatmosphere from 800° C. to 1,000° C. in the firing of the formedhoneycomb material is conducted at such a temperature elevation ratethat the difference of the temperature of the central portion of theformed honeycomb material from the temperature of the firing atmosphereis kept in a range of −40° C. to +40° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relation between the time of firing, thetemperature of central portion of formed honeycomb material, and thetemperature of firing atmosphere when a formed honeycomb material hasbeen fired at a given temperature elevation rate.

FIG. 2 is a graph showing relations between the time of firing, thetemperature of central portion of formed honeycomb material, and thetemperature of firing atmosphere when formed honeycomb materials havebeen fired at given temperature elevation rates.

FIG. 3 is a schematic drawing explaining a “honeycomb shape” in the caseof honeycomb structure.

FIG. 4(a) to FIG. 4(c) are drawings showing the steps of the method fordetermining the temperature elevation rate of firing atmosphere.

BEST MODE FOR CARRYING OUT THE INVENTION

In developing the method of the present invention for firing of ceramichoneycomb structure, the present inventor first made a study on theinfluence, in firing a formed honeycomb material (hereinafter referredsimply to as “formed material”) in a firing atmosphere, of thetemperature elevation rate of the firing atmosphere in a temperaturerange at which the binder contained in the formed material burns, on thegeneration of firing defects such as cracks and the like in the formedmaterial.

To begin with, the graph shown in FIG. 1 indicates changes with time, ofthe temperature of the central portion of formed material and thetemperature of the firing atmosphere (this is roughly equal to thetemperature of the peripheral portion of formed material) when a formedmaterial having a given apparent volume has been fired at differenttemperature elevation rates. It is appreciated from this graph that asthe temperature elevation rate in the temperature range of binderburning is larger, the binder contained in the formed material burnsrapidly and, as a result, the difference between the temperature of thecentral portion of formed material and the temperature of the peripheralportion of formed material (i.e. the temperature of the firingatmosphere) is larger.

The graph shown in FIG. 2 indicates changes with time, of thetemperature of the central portion of formed material and thetemperature of the firing atmosphere (this is roughly equal to thetemperature of the peripheral portion of formed material) when formedmaterials having different apparent volumes have been fired at a giventemperature elevation rate. It is appreciated from this graph that asthe apparent volume of formed material is larger, the absolute amount ofthe binder contained in the formed material is larger and the binderburns rapidly and, as a result, the difference between the temperatureof the central portion of formed material and the temperature of theperipheral portion of formed material (i.e. the temperature of firingatmosphere) is larger. Therefore, it is easily anticipated that theabove-mentioned phenomenon that as the temperature elevation rate in thetemperature range of binder burning is larger, the difference betweenthe temperature of the central portion of formed material and thetemperature of the peripheral portion of formed material is larger, ismore striking as the apparent volume of formed material is larger.

It may be thought from the above findings that the only way to preventthe generation of firing defects such as crack and the like is tominimize the temperature elevation rate in the temperature range ofbinder burning as shown in the above-mentioned Patent Literature 2[employ +2 to +4 (° C./hr) in the case of FIG. 1] to allow the burningof binder to proceed mildly and conduct firing so that there arises notemperature difference between the central portion of formed materialand the peripheral portion of formed material.

However, a study by the present inventor revealed that by makingextremely large the temperature elevation rate of the firing atmospherein the temperature range of binder burning so that the temperaturechange of the firing atmosphere can follow the rapid temperatureincrease of the central portion of formed material caused by the burningof binder, the temperature difference between the central portion offormed material and the peripheral portion of formed material can bemade small unexpectedly and thus the generation of firing defects suchas crack and the like can be prevented.

Hence, in the present invention, it was employed that, in firing aformed honeycomb material in a firing atmosphere, the heating of thefiring atmosphere from a temperature (180 to 300° C.) at which theorganic binder contained in the formed honeycomb material begins toburn, to 300° C. is conducted at a temperature elevation rate of +25 (°C./hr) or more and also at such a temperature elevation rate that thedifference of the temperature of the central portion of the formedhoneycomb material from the temperature of the firing atmosphere is keptin a given temperature range.

By doing so, the temperature difference between the central portion offormed material and the peripheral portion of formed material can bemade small; as a result, the generation of firing defects such as crackand the like can be prevented effectively, and the temperature elevationrate in the temperature range of binder burning can be increasedstrikingly and the conventional firing time can be shortened strikingly.

The modes for carrying out the methods of the present invention forfiring of ceramic honeycomb structure are specifically described below.

(1) Application Target of the Invention

To begin with, description is made on the formed honeycomb materialwhich is the application target of the firing method of the presentinvention. The “formed honeycomb material” which is the applicationtarget of the firing method of the present invention, is a so-callednon-fired honeycomb structure and is obtained by mixing and kneading araw material for aggregate particles, water and an organic binder toform a puddle, forming this puddle into a honeycomb shape and drying thehoneycomb shape.

The aggregate particles are particles which become the main component ofa ceramic honeycomb structure (sintered material) to be obtained, andthe raw material for aggregate particles is a substance which is the rawmaterial for aggregate particles. The raw material for aggregateparticles is not particularly restricted, but there can be mentioned,for example, a raw material to become cordierite, mullite, alumina,aluminum titanate, lithium aluminum silicate, silicon carbide andsilicon nitride.

Incidentally, the raw material to become cordierite means a substancewhich is converted into cordierite upon firing, and there can bementioned, for example, a mixture of talc, kaolin, alumina, silica, etc.wherein they are mixed so as to give, after firing, a theoreticalcomposition (2MgO.2Al₂O₃.5SiO₂) of cordierite. Specifically, there canbe used suitably a mixture of talc, kaolin, alumina, silica, etc.wherein they are mixed so as to give, after firing, a composition ofSiO₂=42 to 56% by mass, Al₂O₃=30 to 45% by mass and MgO=12 to 16% bymass.

The organic binder is an additive which becomes a gel in the formedmaterial before firing (i.e. puddle) and functions as a reinforcingagent for maintenance of the mechanical strength of formed material.Therefore, as the organic binder, there can be suitably used organichigh-molecular substances which can gel in the formed material (puddle),such as hydroxypropyl methyl cellulose, methyl cellulose, hydroxyethylcellulose, carboxymethyl cellulose, polyvinyl alcohol and the like.

Incidentally, the puddle ordinarily contains the above-mentioned rawmaterial for aggregate particles, water and the above-mentioned organicbinder; however, it may as necessary contain other additives, forexample, a hole-making agent and a dispersing agent.

The hole-making agent is an additive which burns and forms pores whenthe formed material is fired, thereby increases the porosity of theformed material and gives a porous honeycomb structure of high porosity.Therefore, as the hole-making agent, there can be mentioned organicsubstances which burn and disappear when the formed material is fired,such as carbon (e.g. graphite), wheat flour, starch, phenolic resin,poly(methyl methacrylate), polyethylene, polyethylene terephthalate andthe like. Of these, a micro-capsule made of a foamed resin (e.g. anacrylic resin-based micro-capsule) can be used particularly preferably.Being hollow, the micro-capsule made of a foamed resin has advantages ofbeing able to give a porous honeycomb structure of high porosity withaddition of a small amount and, moreover, of being low in the generationof heat during firing and being able to reduce the generation of thermalstress.

The dispersing agent is an additive for promoting the dispersion of rawmaterial for aggregate particles in water (dispersing medium). As thedispersing medium, there can be used, for example, ethylene glycol,dextrin, fatty acid soap and polyalcohol.

The raw material for aggregate particles, water, organic binder, etc.are mixed and kneaded by, for example, a vacuum pug mill and can be madeinto a puddle having an appropriate viscosity. The puddle is formed intoa honeycomb shape and the honeycomb shape is dried to obtain a formedhoneycomb material.

The “honeycomb shape” referred to in this specification means a shapesuch as shown in the honeycomb structure 1 of FIG. 3, i.e. a shape inwhich a plurality of cells 3 to become passages for fluid are formed bybeing divided by extremely thin partition walls 4. The overall shape ofthe formed honeycomb material is not particularly restricted and therecan be mentioned, for example, a cylinder shown in FIG. 3, a tetragonalprism and a triangular prism. There is no particular restriction,either, as to the cell shape of the formed honeycomb material (the cellshape in a section normal to the direction of cell formation) and therecan be mentioned, for example, a tetragonal cell shown in FIG. 3, ahexagonal cell and a triangular cell.

The forming can be conducted by a known conventional forming method suchas extrusion forming, injection forming, press forming or the like. Ofthese, there can be preferably used extrusion forming wherein a puddleprepared as above is extrusion-formed using a die having a desired cellshape, a desired partition wall thickness and a desired cell density.Also, the drying can be conducted by a known conventional drying methodsuch as hot-air drying, micro-wave drying, dielectric drying,reduced-pressure drying, vacuum drying, freeze-drying or the like. Ofthese, there is preferred a combined drying method of hot-air drying andmicro-wave drying or dielectric drying because it can dry the wholeportion of the formed honeycomb material rapidly and uniformly.

Incidentally, in the present firing method, a large-sized formedhoneycomb material having an apparent volume of 5 liters or more is anapplication target. Such a large-sized formed honeycomb material whichcontains a large absolute amount of an organic binder, is characterizedin that the temperature increase in its central portion is sharp owingto the burning of the organic binder and further in that the heatgenerated in the formed material is hardly dissipated outside the formedmaterial. Therefore, in the large-sized formed honeycomb material, thetemperature difference between the central portion of formed materialand the peripheral portion of formed material tends to be larger than insmall-sized formed honeycomb materials; generation of firing defectssuch crack and the like is very striking; and the meritorious effects ofthe present firing method appear at a higher level.

Incidentally, the “apparent volume” referred to in this specificationmeans a volume containing even the cell spaces of formed honeycombmaterial. For example, a honeycomb structure having an outer diameter of250 mm and a length of 300 mm has an apparent volume of 15 litersregardless of its cell structure.

(2) First Invention (Firing in Temperature Range of Binder Burning)

The firing method according to the first invention comprises firing theabove-mentioned formed honeycomb material in a firing atmosphere whichis heated at a given temperature elevation rate, to obtain a ceramichoneycomb structure, and is characterized in that the heating of thefiring atmosphere in the temperature range (180 to 300° C.) of binderburning is conducted at a controlled temperature elevation rate.

As described previously, the generation of firing defects such as crackand the like, taking place during the firing of formed material iscaused by the large temperature difference between the central portionof formed material where a sharp temperature increase takes place owingto the burning of the binder contained in the formed material and theperipheral portion of formed material whose temperature changeapproximately follows the temperature change of the firing atmosphere.Therefore, in the firing method of the present invention, the heating ofthe firing atmosphere in the temperature range of binder burning isconducted at an extremely high temperature elevation rate so that thetemperature increase of the firing atmosphere follows the sharptemperature increase of the central portion of formed material broughtabout by the burning of binder. Specifically, the heating of the firingatmosphere from a temperature (180 to 300° C.) at which the organicbinder contained in the formed honeycomb material begins to burn, to300° C. is conducted at a temperature elevation rate of +25 (° C./hr) ormore.

In the firing method according to the first invention, the heating ofthe firing atmosphere is conducted at a controlled temperature elevationrate in a temperature range from the temperature at which the organicbinder contained in the formed honeycomb material begins to burn (thestarting temperature of binder burning), to 300° C. The reason why thestarting point of the controlled temperature elevation rate has beenselected at the starting temperature of binder burning, is that thesharp temperature increase of the central portion of formed material isinitiated by the burning of the organic binder; meanwhile, the reasonwhy the end point of the controlled temperature elevation rate has beenselected at 300° C., is that, with ordinary organic binders, the burningis complete up to 300° C. Incidentally, the starting temperature ofbinder burning is a temperature in a range of 180 to 300° C. although itvaries depending upon the kind of the organic binder used.

In the firing method according to the first invention, the temperatureelevation rate of the firing atmosphere is made extremely high at +25 (°C./hr) or more in order for the firing atmosphere to follow the sharptemperature increase of the central portion of formed material, broughtabout by the burning of binder. Thereby, firing defects such as crackand the like can be prevented effectively and, moreover, the firing timecan be shortened strikingly as compared with the conventional firingtime. Although there is a difference depending upon the conditions offormed material such as apparent volume and the like, when thetemperature elevation rate of the firing atmosphere is less than +25 (°C./hr), there may be a case in which the temperature of the firingatmosphere (i.e. the temperature of the peripheral portion of formedmaterial) is unable to follow the sharp temperature increase of thecentral portion of formed material, brought about by the burning ofbinder, the temperature difference between the central portion of formedmaterial and the peripheral portion of formed material becomes large,and firing defects such as crack and the like generate. Further, whenthe temperature elevation rate of the firing atmosphere is decreasedextremely, the time of firing becomes very long although the firingdefects such as crack and the like may be prevented.

As described above, in the firing method according to the firstinvention, it is necessary to control the temperature elevation rate ofthe firing atmosphere at +25 (° C./hr) or more in a temperature rangefrom the starting temperature of binder firing to 300° C.; and it isfurther necessary that the temperature elevation rate is such atemperature elevation rate that the difference of the temperature of thecentral portion of formed material from the temperature of the firingatmosphere is kept in a given temperature range.

This is because, even when the temperature elevation rate of the firingatmosphere is set at +25 (° C./hr) or more, there may be a case that thetemperature increase of the firing atmosphere (i.e. the peripheralportion of formed material) is not sufficient and is unable to followthe sharp temperature increase of the central portion of formedmaterial, or, the temperature increase of the firing atmosphere isexcessive and exceeds the temperature increase of the central portion offormed material, depending upon various conditions such as the structureof formed material (e.g. apparent volume, partition wall thickness andcell density), the material constituting the formed material (e.g. kindof raw material for aggregate particles, and kind and amount of binder),the oxygen concentration during firing, and the like.

The allowable range of the difference of the temperature of the centralportion of formed material from the temperature of the firing atmosphere(i.e. the temperature of the peripheral portion of formed material)varies depending upon the partition wall thickness of the formedmaterial. The reason is that when the partition wall thickness is large,the formed material has high mechanical strengths and therefore hardlygenerates firing defects such as crack and the like even though thetemperature difference between the central portion of formed materialand the peripheral portion of formed material is relatively large, whilewhen the partition wall thickness is small, the formed material has lowmechanical strengths and therefore firing defects such as crack and thelike are unpreventable unless the temperature difference between thecentral portion of formed material and the peripheral portion of formedmaterial is made small. Specifically, when the formed honeycomb materialhas a partition wall thickness of 90 μm or more, it is sufficient toemploy such a temperature elevation rate (of the firing atmosphere) thatthe difference of the temperature of the central portion of formedmaterial from the temperature of the firing atmosphere is kept in arange of −80 to +80° C.; however, when the formed honeycomb material hasa partition wall thickness of less than 90 μm, there is required such atemperature elevation rate that the difference of the temperature of thecentral portion of formed material from the temperature of the firingatmosphere is kept in a range of −40 to +50°C.

“Such a temperature elevation rate that the difference of thetemperature of the central portion of formed material from thetemperature of the firing atmosphere is kept in a given temperaturerange” varies depending upon various conditions such as the structure offormed material (e.g. apparent volume, partition wall thickness and celldensity), the material constituting the formed material (e.g. kind ofraw material for aggregate particles, and kind and quantity of binder),and the oxygen concentration during firing. However, the temperatureelevation rate can be determined by, for example, the method shownbelow.

As shown in FIG. 4(a), first, a formed material to be fired actually (aformed material to be tested) is subjected to test firing under theconditions intended to be employed actually, in a firing atmospherewhich is heated at a tentative temperature elevation rate [+8 (° C./hr)in the case of FIG. 4(a)]. The temperatures of the central portion ofthe formed material during the test firing at the tentative temperatureelevation rate are measured using a thermocouple or the like andrecorded against time to prepare a temperature curve of the centralportion.

As is clear from the temperature curve of the central portion, shown inFIG. 4(a), the temperature of the central portion of the formed materialincreases sharply from the starting temperature of binder burning andthe difference of the temperature of the central portion from thetemperature of the firing atmosphere reaches the maximum. Hence, asshown in FIG. 4(b), an actual temperature elevation rate of the firingatmosphere [at +30 (° C./hr) in the case of FIG. 4(b)] is determined soas to follow the temperature curve of the central portion. When actualfiring is conducted using the thus-determined temperature elevationrate, the difference of the temperature of the central portion of formedmaterial from the temperature of the firing atmosphere is kept in agiven temperature range, as shown in FIG. 4(c).

Incidentally, the degree of the burning of the organic binder isaffected also by the level of the tentative temperature elevation rateset. Therefore, when the tentative temperature elevation rate has beenset inappropriately, there may be a case that the difference of thetemperature of the central portion of formed material from thetemperature of the firing atmosphere is not kept in a given temperaturerange. In such a case, the tentative temperature elevation rate isreset, the test firing is conducted again, and the temperature curve ofthe central portion is prepared again. Based on the newly-obtainedtemperature curve of the central portion and according to the samemanner as above, an actual temperature elevation rate of firingatmosphere can be determined.

(3) Second Invention (Firing in Talc Dehydration Temperature Range)

The firing method according to the second invention comprises firing theabove-mentioned formed honeycomb material in a firing atmosphere whichis heated at a given temperature elevation rate, to obtain a ceramichoneycomb structure, and is characterized particularly in that, when theraw material for aggregate particles is a raw material to becomecordierite, the temperature elevation rate of the firing atmosphere intalc dehydration temperature range (800 to 1,000° C.) is controlled.

The present inventor made a study on the problem of generation of firingdefects such as crack and the like during firing of formed material. Asa result, the present inventor found a fact that firing defects such ascrack and the like may generate in a temperature range of 800 to 1,000°C., as well as in the above-mentioned temperature range (180 to 300° C.)of binder burning.

This generation of firing defects such as crack and the like in atemperature range of 800 to 1,000° C. is a phenomenon particular to acase that the raw material for aggregate particles, contained in theformed material is a raw material to become cordierite, and it is causedbecause, in the temperature range of 800 to 1,000° C., a temperaturedifference becomes large between the central portion of formed materialwhere a sharp temperature decrease takes place owing to the endothermicreaction brought about by dehydration from talc (a component of the rawmaterial to become cordierite) and the peripheral portion of formedmaterial where a temperature change takes place so as to approximatelyfollow the temperature of the firing atmosphere.

Therefore, in the firing method according to the second invention, thetemperature elevation rate of the firing atmosphere in a temperaturerange of 800 to 1,000° C. (the temperature range of talc dehydration) issuppressed low at a given level or below (that is, slow firing isconducted) so as to correspond to the sharp temperature decrease of thecentral portion of formed material, brought about by the endothermicreaction. Specifically, the temperature elevation rate of the firingatmosphere from 800 to 1,000° C. is set at +40 (° C./hr) or less.

In the firing method according to the second invention, the temperatureelevation rate of the firing atmosphere is supressed low at °40 (°C./hr) or less so as to correspond to the sharp temperature decrease ofthe central portion of formed material, brought about by the endothermicreaction. Thereby, firing defects such as crack and the like can beprevented effectively. When the temperature elevation rate of the firingatmosphere is controlled above +40 (° C./hr), the temperature of thefiring atmosphere (i.e. the temperature of the peripheral portion offormed material) increases although the temperature of the centralportion of formed material decreases sharply owing to the endothermicreaction; as a result, the temperature difference between the centralportion of formed material and the peripheral portion of formed materialbecomes large and there may be generation of firing defects such ascrack and the like.

As described above, in the firing method according to the secondinvention, it is necessary to control the temperature elevation rate ofthe firing atmosphere at +40 (° C./hr) or less in a temperature rangefrom 800 to 1,000° C.; and it is further necessary that the temperatureelevation rate is such a temperature elevation rate that the differenceof the temperature of the central portion of formed material from thetemperature of the firing atmosphere is kept in a given temperaturerange.

This is because even when the temperature elevation rate of the firingatmosphere is made at +40 (° C./hr) or less, there may be a case thatthe temperature increase of the firing atmosphere becomes excessive andis unable to follow the temperature decrease of the central portion offormed material, depending upon various conditions such as the structureof formed material (e.g. apparent volume, partition wall thickness andcell density), the material constituting the formed material (e.g. kindof raw material for aggregate particles, and kind and amount of binder),the oxygen concentration during firing, and the like.

The allowable range of the difference of the temperature of the centralportion of formed material from the temperature of the firing atmosphere(i.e. the temperature of the peripheral portion of formed material)varies depending upon the partition wall thickness of the formedmaterial. The reason is that when the partition wall thickness is large,the formed material has high mechanical strengths and therefore hardlygenerates firing defects such as crack and the like even if thetemperature difference between the central portion of formed materialand the peripheral portion of formed material is relatively large, whilewhen the partition wall thickness is small, the formed material has lowmechanical strengths and therefore firing defects such as crack and thelike are unpreventable unless the temperature difference between thecentral portion of formed material and the peripheral portion of formedmaterial is made small. Specifically, when the formed honeycomb materialhas a partition wall thickness of 90 μm or more, there is preferred sucha temperature elevation rate (of the firing atmosphere) that thedifference of the temperature of the central portion of formed materialfrom the temperature of the firing atmosphere is kept in a range of −60to +60° C.; however, when the formed honeycomb material has a partitionwall thickness of less than 90 μm, there is preferred such a temperatureelevation rate that the difference of the temperature of the centralportion of formed material from the temperature of the firing atmosphereis kept in a range of −40 to +40° C.

“Such a temperature elevation rate that the difference of thetemperature of the central portion of formed material from thetemperature of the firing atmosphere is kept in a given temperaturerange” varies depending upon various conditions such as the structure offormed material (e.g. apparent volume, partition wall thickness and celldensity), the material constituting the formed material (e.g. kind ofraw material for aggregate particles, and kind and quantity of binder),and the oxygen concentration during firing. As in the case of the firingmethod according to the first invention, a formed material to be firedactually (a formed material to be tested) is subjected to test firingunder the conditions intended to be employed actually, in a firingatmosphere which is heated at a tentative temperature elevation rate;and the temperatures of the central portion of the formed materialduring the test firing at the tentative temperature elevation rate aremeasured using a thermocouple or the like and recorded against time toprepare a temperature curve of the central portion. If the difference ofthe temperature of the central portion of formed material indicated bythe temperature curve of the central portion, from the temperature ofthe firing atmosphere is kept in a given temperature range, thetentative temperature elevation rate may be employed as an actualtemperature elevation rate of firing atmosphere. When actual firing isconducted using the thus-determined temperature elevation rate, thedifference of the temperature of the central portion of formed materialfrom the temperature of the firing atmosphere is kept in a giventemperature range.

Incidentally, the degree of the endothermic reaction brought about bythe dehydration from talc is affected also by the level of the tentativetemperature elevation rate set. Therefore, when the tentativetemperature elevation rate has been set inappropriately, there may be acase that the difference of the temperature of the central portion offormed material from the temperature of the firing atmosphere is notkept in a given temperature range. In such a case, the tentativetemperature elevation rate is reset, the test firing is conducted again,and the temperature curve of the central portion is prepared again. Byrepeating the above procedure until the difference of the temperature ofthe central portion of formed material indicated by the temperaturecurve of the central portion, from the temperature of the firingatmosphere comes to fall in a given temperature range, an actualtemperature elevation rate of firing atmosphere can be determined.

EXAMPLES

The present invention is described more specifically below by way ofExamples. However, the present invention is in no way restricted bythese Examples. Incidentally, in the following Examples and ComparativeExamples, there was used, as the average particle diameter of the usedraw material for aggregate particles, a 50% particle diameter obtainedby a measurement using a particle size distribution tester of X-raytransmission type (for example, Sedigraph 5000-02 produced by ShimadzuCorporation) which conducts detection by the X-ray transmission methodbased on the Stokes' liquid phase sedimentation method.

[Production of Formed Honeycomb Materials]

(Formed Material A)

As a raw material for aggregate particles, there was prepared a rawmaterial to become cordierite, by mixing 40% by mass of talc (averageparticle diameter: 10 μm), 35% by mass of kaolin (average particlediameter: 10 μm), 10% by mass of alumina (average particle diameter: 6μm), 10% by mass of aluminum hydroxide (average particle diameter: 1 μm)and 5% by mass of silica (average particle diameter: 5 μm).

To 100 parts by mass of the raw material for aggregate particles wereadded 3 parts by mass of methyl cellulose as an organic binder, 0.5 partby mass of potassium laurate as a dispersing agent (a surfactant) and 32parts by mass of water. Mixing and kneading was conducted using a vacuumpug mill to prepare a puddle.

The puddle was extrusion-formed into a honeycomb shape using a diehaving the below-mentioned cell shape, partition wall thickness and celldensity. The honeycomb shape was dried by a combined drying method ofhot-air drying and micro-wave drying to obtain a formed honeycombmaterial (formed material A). The overall shape of the formed honeycombmaterial (formed material A) was such that the end face (cell-openingface) shape was a circle having an outer diameter of 196 mm, the lengthwas 182 mm, the cell shape was a square of 1.3 mm×1.3 mm, the partitionwall thickness was 154 μm, the cell density was about 59 cells/cm², thenumber of total cells was 18,000, and the apparent volume was 5 liters.

(Formed Material B)

As a raw material for aggregate particles, there was prepared a rawmaterial to become cordierite, by mixing 40% by mass of talc (averageparticle diameter: 7 μm), 19% by mass of kaolin (average particlediameter: 7 μm), 14% by mass of alumina (average particle diameter: 5μm), 15% by mass of aluminum hydroxide (average particle diameter: 1 μm)and 12% by mass of silica (average particle diameter: 3 μm).

To 100 parts by mass of the raw material for aggregate particles wereadded 4 parts by mass of methyl cellulose as an organic binder, 0.5 partby mass of potassium laurate as a dispersing agent (a surfactant) and 34parts by mass of water. Mixing and kneading was conducted using a vacuumpug mill to prepare a puddle.

The puddle was extrusion-formed into a honeycomb shape using a diehaving the below-mentioned cell shape, partition wall thickness and celldensity. The honeycomb shape was dried by a combined drying method ofhot-air drying and micro-wave drying to obtain a formed honeycombmaterial (formed material B). The overall shape of the formed honeycombmaterial (formed material B) was such that the end face (cell-openingface) shape was a circle having an outer diameter of 277 mm, the lengthwas 157 mm, the cell shape was a square of 1.3 mm×1.3 mm, the partitionwall thickness was 78 μm, the cell density was about 58 cells/cm², thenumber of total cells was about 35,000, and the apparent volume was 9liters.

(Formed Material C)

As a raw material for aggregate particles, there was prepared a rawmaterial to become cordierite, by mixing 40% by mass of talc (averageparticle diameter: 7 μm), 19% by mass of kaolin (average particlediameter: 7 μm), 14% by mass of alumina (average particle diameter: 5μm), 15% by mass of aluminum hydroxide (average particle diameter: 1 μm)and 12% by mass of silica (average particle diameter: 3 μm).

To 100 parts by mass of the raw material for aggregate particles wereadded 4 parts by mass of methyl cellulose as an organic binder, 0.5 partby mass of potassium laurate as a dispersing agent (a surfactant) and 34parts by mass of water. Mixing and kneading was conducted using a vacuumpug mill to prepare a puddle.

The puddle was extrusion-formed into a honeycomb shape using a diehaving the below-mentioned cell shape, partition wall thickness and celldensity. The honeycomb shape was dried by a combined drying method ofhot-air drying and micro-wave drying to obtain a formed honeycombmaterial (formed material C). The overall shape of the formed honeycombmaterial (formed material C) was such that the end face (cell-openingface) shape was a circle having an outer diameter of 296 mm, the lengthwas 209 mm, the cell shape was a square of 1.3 mm×1.3 mm, the partitionwall thickness was 104 μm, the cell density was about 58 cells/cm², thenumber of total cells was about 40,000, and the apparent volume was 14liters.

Examples 1-1 to 1-4 and Comparative Examples 1-1 to 1-14

The formed honeycomb materials (formed materials A to C) were fired eachunder the conditions shown in Table 1 to investigate the influence ofthe temperature elevation rate of firing atmosphere in the temperaturerange (180 to 300° C.) of binder burning, on the defects of firing suchas crack and the like and the time of firing. Incidentally, the firingwas conducted using a batch type oven.

In Table 1, the temperature elevation rate of firing atmosphere in thetemperature range of binder burning was expressed as “180 to 300° C.temperature elevation rate”; the maximum temperature difference in thetemperature range of binder burning, of the temperature of centralportion of formed material from the temperature of firing atmosphere wasexpressed as “180 to 300° C. maximum temperature difference”; thetemperature elevation rate of firing atmosphere in the temperature rangeof talc dehydration was expressed as “800 to 1,000 temperature elevationrate”; and the maximum temperature difference in the temperature rangeof talc dehydration, of the temperature of central portion of formedmaterial from the temperature of firing atmosphere was expressed as “800to 1,000° C. maximum temperature difference”.

In the column of “180 to 300° C. temperature elevation rate”, “8→30”means that the temperature elevation rate from 180° C. to the startingtemperature of binder burning was +8 (° C./hr) and the temperatureelevation rate from the starting temperature of binder burning to 300°C. was +30 (° C./hr). “8→60” has a similar meaning.

The “180 to 300° C. maximum temperature difference” was calculated by,in firing any formed material actually, measuring the temperature of thecentral portion of the formed material and the temperature of the firingatmosphere used, using a R thermocouple. Incidentally, the allowablerange of the maximum temperature difference is in a range of −80 to +80°in the formed materials A and C each having a partition wall thicknessof 90 μm or more and in a range of −40 to +50° C. in the formed materialB having a partition wall thickness of less than 90 μm.

In all of Examples 1-1 to 1-4 and Comparative Examples 1-1 to 1-14, the“800 to 1,000° C. temperature elevation rate” was +25 (° C./hr). The“800 to 1,000° C. maximum temperature difference” was calculated by, infiring any formed material actually, measuring the temperature of thecentral portion of the formed material and the temperature of the firingatmosphere used, using a R thermocouple. Incidentally, the allowablerange of the maximum temperature difference is preferred to be in arange of −60 to +60° C. in the formed materials A and C each having apartition wall thickness of 90 μm or more and in a range of −40 to +40°C. in the formed material B having a partition wall thickness of lessthan 90 μm; and these allowable ranges were satisfied in all of Examples1-1 to 1-4 and Comparative Examples 1-1 to 1-14.

In Table 1, “product yield” indicates the proportion of acceptedproducts free from firing defect (e.g. crack) to total formed materialsfired when 20 samples of each of the formed honeycomb materials (formedmaterials A to C) were fired under the conditions shown in Table 1.“Evaluation of yield” was expressed by ◯ when the product yield was 95%or more, by Δ when the product yield was 80% to less than 95%, and by Xwhen the product yield was less than 80%.

In Table 1, “firing time” was defined as a time taken from start offiring to completion of firing, in the firing program set for the batchtype oven. In this case, the start of firing was a point at which, afterthe formed material was placed in the batch type oven, the temperatureof firing atmosphere passed 100° C. in the temperature elevation step;and the completion of firing was a point at which the temperature offiring atmosphere reached 1000° C. in the temperature decrease (cooling)step. TABLE 1 180 to 300° C. 180 to 300° C. 800 to 1000° C. 800 to 1000°C. temperature maximum temperature maximum Kind of elevation temperatureelevation temperature Product Time of formed rate difference ratedifference yield Evaluation firing material (° C./hr) (° C) (° C./hr) (°C.) (%) of yield (hr) Comp. Example 1-1 Formed material A  4 17 25 −46100

94 Comp. Example 1-2 Formed material A  8 41 25 −45 100

79 Comp. Example 1-3 Formed material A 13 63 25 −46 100

73 Comp. Example 1-4 Formed material A 20 84 25 −45 90 Δ 70 Example 1-1Formed material A 8→30 50 25 −45 100

68 Example 1-2 Formed material A 8→60 23 25 −44 100

66 Comp. Example 1-5 Formed material B  4 45 25 −26 100

94 Comp. Example 1-6 Formed material B  8 52 25 −26 50 X 79 Comp.Example 1-7 Formed material B 13 86 25 −27 0 X 73 Comp. Example 1-8Formed material B 20 73 25 −24 0 X 70 Example 1-3 Formed material B 8→3027 25 −26 100

68 Comp. Example 1-9 Formed material B 8→60 −43 25 −26 25 X 66 Comp.Example 1-10 Formed material C  4 68 25 −37 100

94 Comp. Example 1-11 Formed material C  8 87 25 −38 50 X 79 Comp.Example 1-12 Formed material C 13 102 25 −36 0 X 73 Comp. Example 1-13Formed material C 20 150 25 −37 0 X 70 Comp. Example 1-14 Formedmaterial C 8→30 111 25 −38 75 X 68 Example 1-4 Formed material C 8→60 5625 −37 100

66(Evaluation)

In each of Examples 1-1 to 1-4, the temperature elevation rate from thestarting temperature of binder burning to 300° C. was +25 (° C./hr) ormore and the maximum temperature difference in 180 to 300° C. was in thepredetermined range; therefore, there was no firing defect such as crackor the like, the product yield was good, and the firing time wassignificantly short compared with the conventional firing time. In eachof Comparative Examples 1-1 to 1-3, Comparative Example 1-5 andComparative Example 1-10, the temperature elevation rate was +4 (°C./hr), the maximum temperature difference in 180 to 300° C. was in thepredetermined range and, therefore, there was no firing defect such ascrack or the like and the product yield was good; however, the firingtime was very long.

In each of Comparative Example 1-4, Comparative Examples 1-6 to 1-8 andComparative Examples 1-11 to 1-13, the maximum temperature difference in180 to 300° C. was not in the predetermined range because thetemperature elevation rate was less than +25 (° C./hr); therefore, therewere firing defects such as crack and the like and the product yield waslow.

In each of Comparative Example 1-9 and Comparative Example 1-14, thetemperature elevation rate from the starting temperature of binderburning to 300° C. was +25 (° C./hr) or more but the maximum temperaturedifference in 180 to 300° C. was not in the predetermined range;therefore, there were firing defects such as crack and the like and theproduct yield was low.

Examples 2-1 to 2-9 and Comparative Examples 2-1 to 2-6

The formed honeycomb materials (formed materials A to C) were fired eachunder the conditions shown in Table 2 to investigate the influence ofthe temperature elevation rate of firing atmosphere in the temperaturerange (800 to 1,000° C.) of talc dehydration, on the defects of firingsuch as crack and the like and the time of firing. Incidentally, thefiring was conducted using a batch type oven.

In Table 2, the temperature elevation rate of firing atmosphere in thetemperature range of binder burning was expressed as “180 to 300° C.temperature elevation rate”; the maximum temperature difference in thetemperature range of binder burning, of the temperature of centralportion of formed material from the temperature of firing atmosphere wasexpressed as “180 to 300° C. maximum temperature difference”; thetemperature elevation rate of firing atmosphere in the temperature rangeof talc dehydration was expressed as “800 to 1,000° C. temperatureelevation rate”; and the maximum temperature difference in thetemperature range of talc dehydration, of the temperature of centralportion of formed material from the temperature of firing atmosphere wasexpressed as “800 to 1,000° C. maximum temperature difference”.

In all of Examples 2-1 to 2-9 and Comparative Examples 2-1 to 2-6, the“180 to 300° C. temperature elevation rate” was +4 (° C./hr). The “180to 300° C. maximum temperature difference” was calculated by, in firingany formed material actually, measuring the temperature of the centralportion of the formed material and the temperature of the firingatmosphere used, using a R thermocouple. Incidentally, the allowablerange of the maximum temperature difference is required to be in a rangeof −80 to +80° C.in the formed materials A and C each having a partitionwall thickness of 90 μm or more and in a range of −40 to +50° C. in theformed material B having a partition wall thickness of less than 90 μm;and these allowable ranges were satisfied in all of Examples 2-1 to 2-9and Comparative Examples 2-1 to 2-6.

The “800 to 1,000° C. maximum temperature difference” was calculated by,in firing any formed material actually, measuring the temperature of thecentral portion of the formed material and the temperature of the firingatmosphere used, using a R thermocouple. Incidentally, the allowablerange of the maximum temperature difference is preferred to be in arange of −60 to +60° C. in the formed materials A and C each having apartition wall thickness of 90 μm or more and in a range of −40 to +40°C. in the formed material B having a partition wall thickness of lessthan 90 μm.

In Table 2, “product yield” indicates the proportion of acceptedproducts free from firing defect (e.g. crack) to total formed materialsfired when 20 samples of each of the formed honeycomb materials (formedmaterials A to C) were fired under the conditions shown in Table 2.“Evaluation of yield” was expressed by

when the product yield was 95% or more, by Δ when the product yield was80% to less than 95%, and by X when the product yield was less than 80%.

In Table 2, “firing time” was defined as a time taken from start offiring to completion of firing, in the firing program set for the batchtype oven. In this case, the start of firing was a point at which, afterthe formed material was placed in the batch type oven, the temperatureof firing atmosphere passed 100° C. in the temperature elevation step;and the completion of firing was a point at which the temperature offiring atmosphere reached 100° C. in the temperature decrease (cooling)step. TABLE 2 180 to 300° C. 180 to 300° C. 800 to 1000° C. 800 to 1000°C. temperature maximum temperature maximum Kind of elevation temperatureelevation temperature Product Time of formed rate difference ratedifference yield Evaluation firing material (° C./hr) (° C) (° C./hr) (°C.) (%) of yield (hr) Example 2-1 Formed material A 4 18 10 −27 100

106 Example 2-2 Formed material A 4 17 25 −45 100

94 Example 2-3 Formed material A 4 17 40 −60 100

91 Comp. Example 2-1 Formed material A 4 17 60 −74 100

90 Comp. Example 2-2 Formed material A 4 18 90 −98 80 Δ 88 Example 2-4Formed material B 4 44 10 −14 100

106 Example 2-5 Formed material B 4 45 25 −26 100

94 Example 2-6 Formed material B 4 45 40 −39 100

91 Comp. Example 2-3 Formed material B 4 46 60 −52 75 X 90 Comp. Example2-4 Formed material B 4 45 90 −72 0 X 88 Example 2-7 Formed material C 468 10 −21 100

106 Example 2-8 Formed material C 4 69 25 −37 100

94 Example 2-9 Formed material C 4 68 40 −49 100

91 Comp. Example 2-5 Formed material C 4 68 60 −66 90 Δ 90 Comp. Example2-6 Formed material C 4 67 90 −87 0 X 88(Evaluation)

In each of Examples 2-1 to 2-9, the 800 to 1,000° C. temperatureelevation rate was +40 (° C./hr) or less and the 800 to 1,000° C.maximum temperature difference was in the predetermined range;therefore, there was no firing defect such as crack or the like and theproduct yield was good. In each of Comparative Examples 2-1 to 2-6, the800 to 1,000° C. temperature elevation rate exceeded +40 (° C./hr) andconsequently the 800 to 1,000° C. maximum temperature difference was notin the predetermined range; therefore, there were firing defects such ascrack and the like and the product yield was low.

Incidentally, in Comparative Example 2-1, although the 800 to 1,000° C.temperature elevation rate exceeded +40 (° C./hr), there was no firingdefect such as crack or the like and the product yield was good, underthe conditions used. However, since the 800 to 1,000° C. maximumtemperature difference was not in the predetermined range, firingdefects such as crack and the like may generate and the product yieldmay be low when there vary conditions such as the structure of formedmaterial (e.g. apparent volume, partition wall thickness and celldensity), the material constituting the formed material (e.g. kind ofraw material for aggregate particles, and kind and amount of binder),and the concentration of oxygen during firing.

INDUSTRIAL APPLICABILITY

As described above, the method for firing of ceramic honeycomb structureaccording to the present invention can prevent firing defects such ascrack and the like effectively and can give a firing time which issignificantly shorter than the conventional firing time; therefore, themethod can be suitably used for production of a ceramic honeycombstructure of low pressure loss and high porosity which is useful as adust-collecting filter, particularly as a diesel particulate filter.

1. A method for firing of ceramic honeycomb structure, which comprisesmixing and kneading at least a raw material for aggregate particles,water and an organic binder to form a puddle, forming the puddle into ahoneycomb shape having a plurality of cells (to become passages forfluid) divided by partition walls, drying the honeycomb shape to obtaina formed honeycomb material having an apparent volume of 5 liters ormore and a partition wall thickness of 90 μm or more, and firing theformed honeycomb material in a firing atmosphere which is heated at agiven temperature elevation rate to obtain a ceramic honeycombstructure, wherein, in the firing of the formed honeycomb material inthe firing atmosphere, the heating of the firing atmosphere from atemperature (180 to 300° C.) at which the organic binder contained inthe formed honeycomb material begins to burn, to 300° C. is conducted ata temperature elevation rate of +25 (° C./hr) or more and also at such atemperature elevation rate that the difference of the temperature of thecentral portion of the formed honeycomb material from the temperature ofthe firing atmosphere is kept in a range of −80° C. to +80° C.
 2. Amethod for firing of ceramic honeycomb structure, which comprises mixingand kneading at least a raw material for aggregate particles, water andan organic binder to form a puddle, forming the puddle into a honeycombshape having a plurality of cells (to become passages for fluid) dividedby partition walls, drying the honeycomb shape to obtain a formedhoneycomb material having an apparent volume of 5 liters or more and apartition wall thickness of less than 90 μm, and firing the formedhoneycomb material in a firing atmosphere which is heated at a giventemperature elevation rate to obtain a ceramic honeycomb structure,wherein, in the firing of the formed honeycomb material in the firingatmosphere, the heating of the firing atmosphere from a temperature (180to 300° C.) at which the organic binder contained in the formedhoneycomb material begins to burn, to 300° C. is conducted at atemperature elevation rate of +25 (° C./hr) or more and also at such atemperature elevation rate that the difference of the temperature of thecentral portion of the formed honeycomb material from the temperature ofthe firing atmosphere is kept in a range of −40° C. to +5° C.
 3. Amethod for firing of ceramic honeycomb structure, which comprises mixingand kneading at least a raw material to become cordierite and water toform a puddle, forming the puddle into a honeycomb shape having aplurality of cells (to become passages for fluid) divided by partitionwalls, drying the honeycomb shape to obtain a formed honeycomb materialhaving an apparent volume of 5 liters or more, and firing the formedhoneycomb material in a firing atmosphere which is heated at a giventemperature elevation rate to obtain a ceramic honeycomb structure,wherein, in the firing of the formed honeycomb material in the firingatmosphere, the heating of the firing atmosphere from 800° C. to 1,000°C. is conducted at a temperature elevation rate of +40 (° C./hr) orless.
 4. A method for firing of ceramic honeycomb structure according toclaim 3, wherein, when the formed honeycomb material obtained has apartition wall thickness of 90 μm or more, the heating of the firingatmosphere from 800° C. to 1,000° C. in the firing of the formedhoneycomb material is conducted at such a temperature elevation ratethat the difference of the temperature of the central portion of theformed honeycomb material from the temperature of the firing atmosphereis kept in a range of −60° C. to +60° C.
 5. A method for firing ofceramic honeycomb structure according to claim 3, wherein, when theformed honeycomb material obtained has a partition wall thickness ofless than 90 μm, the heating of the firing atmosphere from 800° C. to1,000° C. in the firing of the formed honeycomb material is conducted atsuch a temperature elevation rate that the difference of the temperatureof the central portion of the formed honeycomb material from thetemperature of the firing atmosphere is kept in a range of −40° C. to+40° C.